Substance P and gastrin releasing peptide are putative neurotransmitters of the central and peripheral nervous system. Peptide neurotransmitters may exert their actions locally within tissues and may be inactivated by membrane-bound peptidases close to or on the target cells. The aim of this proposal is to examine the physiological role of membrane-bound peptidases in terminating the biological actions of neuropeptides. Catabolism of substance P and gastrin releasing peptide will be examined in the stomach because gastric tissue is a rich source of these peptides and because the stomach is an accessible organ for physiological study. Initially, membranes will be prepared from the gastric corpus of the pig stomach by repeated ultracentrifugation. The peptidases will be solubilized in detergents and purified to homogeneity by column chromatography. The purified enzyme will be chemically and enzymatically characterized. Antibodies, both polyclonal and monoclonal, will be raised to the peptidase and used for the localization of the enzyme by radioimmunoassay and innunocytochemistry. The pattern of neuropeptide catabolism in the stomach wall of the conscious pig will be investigated by use of chronically implanted dialysis tubes to collect the catabolic products. The physiologicl role of the enzyme will be assessed by examining the effects of selective enzyme inhibitors on the pattern of catabolism and the biological actions of the neuropeptides in gastric tissues.